It is known in the art to use an apparatus having differential thread fastening means, i.e., threads having different pitches, for clamping a cutting tool. Such an apparatus is shown, for example, in U.S. Pat. No. 5,758,883.
As shown in '883, an apparatus for adjusting the grip of a tubular collet upon a rotary tool is disclosed. The apparatus comprises a holder, a collet, a rotary tool and a rotatable connector member. The connector member has a first thread connection having a first pitch for threadingly engaging the collet, and a second thread connection having a second pitch for threadingly engaging the holder.
When the connector member is rotated, the collet is axially displaced into a tapered portion of the holder. The collet is provided with longitudinally extending slits so that further rotation of the connector member will cause the collet to radially close upon the shaft of the rotary tool and thus clamp the rotary tool within the collet.
In order to operate in an accurate and stable manner, the apparatus of '883 has to be designed to deal with several degrees of freedom, i.e., taking into consideration the dimensions and tolerances of the various parts. A first degree of freedom is defined between the tapered portions of the collet and of the holder. A second degree of freedom is defined in the first thread connection between the connector member and the collet. A third degree of freedom is defined in the second thread connection between the connector member and the holder. A fourth degree of freedom is defined in the slits of the collet. A fifth degree of freedom is defined between the bore of the collet and the shaft of the rotary tool retained therein.
Since there are involved several degrees of freedom between various parts, it negatively affects the possibility to have an accurate and stable clamping. This relates particularly to the slits of the collet and to the space between the bore of the collet and the shaft of the cutting tool. Furthermore, it may negatively affect the rigidity and clamping force by which the shaft of the cutting tool is clamped within the bore of the collet.